The present invention relates generally to the field of electrosurgery and, more particularly, to surgical devices and methods which employ high frequency voltage to treat tissue and other body structures within the body.
The field of electrosurgery includes a number of loosely related surgical techniques which have in common the application of electrical energy to modify the structure or integrity of patient tissue. Electrosurgical procedures usually operate through the application of very high frequency currents to cut or ablate tissue structures, where the operation can be monopolar or bipolar. Monopolar techniques rely on external grounding of the patient, where the surgical device defines only a single electrode pole. Bipolar devices comprise both electrodes for the application of current between their surfaces.
Electrosurgical procedures and techniques are particularly advantageous since they generally reduce patient bleeding and trauma associated with cutting operations. Current electrosurgical device and procedures, however, suffer from a number of disadvantages. For example, conventional electrosurgical cutting devices typically operate by creating a voltage difference between the active electrode and the target tissue, causing an electrical arc to form across the physical gap between the electrode and tissue. At the point of contact of the electric arcs with tissue, rapid tissue heating occurs due to high current density between the electrode and tissue. This high current density causes cellular fluids to rapidly vaporize into steam, thereby producing a "cutting effect" along the pathway of localized tissue heating. Thus, the tissue is parted along the pathway of evaporated cellular fluid, inducing undesirable collateral tissue damage in regions surrounding the target tissue site.
Further, monopolar devices generally direct electric current along a defined path from the exposed or active electrode through the patient's body to the return electrode, which is externally attached to a suitable location on the patient. This creates the potential danger that the electric current will flow through undefined paths in the patient's body, thereby increasing the risk of unwanted electrical stimulation to portions of the patient's body. In addition, since the defined path through the patient's body has a relatively high impedance (because of the large distance or resistivity of the patient's body), large voltage differences must typically be applied between the return and active electrodes in order to generate a current suitable for ablation or cutting of the target tissue. This current, however, may inadvertently flow along body paths having less impedance than the defined electrical path, which will substantially increase the current flowing through these paths, possibly causing damage to or destroying tissue along and surrounding this pathway.
Bipolar electrosurgical devices have an inherent advantage over monopolar devices because the return current path does not flow through the patient. In bipolar electrosurgical devices, both the active and return electrode are typically exposed so that they may both contact tissue, thereby providing a return current path from the active to the return electrode through the tissue. One drawback with this configuration, however, is that the return electrode may cause tissue desiccation or destruction at its contact point with the patient's tissue. In addition, the active and return electrodes are typically positioned close together to ensure that the return current flows directly from the active to the return electrode. The close proximity of these electrodes generates the danger that the current will short across the electrodes, possibly impairing the electrical control system and/or damaging or destroying surrounding tissue.
The use of electrosurgical procedures (both monopolar and bipolar) in electrically conductive environments can be further problematic. For example, many arthroscopic procedures require flushing of the region to be treated with isotonic saline (also referred to as normal saline), both to maintain an isotonic environment and to keep the field of viewing clear. The presence of saline, which is a highly conductive electrolyte, can also cause shorting of the electrosurgical electrode in both monopolar and bipolar modes. Such shorting causes unnecessary heating in the treatment environment and can further cause non-specific tissue destruction.
To overcome the above problems with conventional electrosurgery, improved electrosurgical techniques have been developed using a cold ablation process that employs molecular dissociation or disintegration (rather than thermal evaporation or carbonization) to volumetrically remove body tissue. In these techniques, high frequency voltage is applied to one or more electrode terminal(s) to vaporize an electrically conductive fluid (e.g., gel or isotonic saline) between the electrode terminal(s) and the soft tissue. Within the vaporized fluid, a ionized plasma is formed and charged particles (e.g., electrons) are accelerated towards the tissue to cause the molecular breakdown or disintegration of several cell layers of the tissue. This molecular dissociation is accompanied by the volumetric removal of the tissue. The short range of the accelerated charged particles within the plasma layer confines the molecular dissociation process to the surface layer to minimize damage and necrosis to the underlying tissue. This process can be precisely controlled to effect the volumetric removal of tissue as thin as 10 to 150 microns with minimal heating of, or damage to, surrounding or underlying tissue structures. A more complete description of this phenomena is described in commonly assigned U.S. Pat. No. 5,683,366, the complete disclosure of which is incorporated herein by reference.
This new technology for the electrosurgical removal and/or modification of tissue has, of course, created many new challenges. One such challenge involves excessive power dissipation into the electrically conductive environment surrounding the electrode terminal(s). To perform the above cold ablation technique, conductive fluid is typically either directed to the electrode terminal on the probe (dry fields, such as skin resurfacing) or the conductive fluid is present at the target site during operation (wet fields, such as arthroscopy). When the RF probe is not directly engaging body tissue but still immersed in conductive fluid, relatively large amounts of current may flow from the electrode terminate into the conductive environment. This excessive power wastes energy from the power supply, and may cause it to overheat, which degrades power supply performance and may cause unwanted collateral tissue damage.
Another challenge in electrosurgical procedures involves soft tissue removal in confined spaces where accidental contact is likely to occur between an electrode on the surgical instrument and a low impedance object, such as a metallic endoscope or an anchor. Arthroscopic procedures, laproscopic procedures, and the like are often conducted in confined areas such as the synovial sac of the knee or similar body enclosures. Several instruments, such as an endoscope for visualization and one or two cannulas for surgical access, are often required to perform such surgeries. In arthroscopic procedures, it may also be necessary to remove soft tissue close to an anchor without delivering electrical energy to the anchor. The low impedance of a metallic anchor or the shaft of an endoscope, as compared to the impedance of the surrounding body tissue, may cause a spark or excessive pulse/power drawdown from the instrument. The spark may cause undesired tissue necrosis and also permanently damage surgical equipment such as the endoscope. Delivering current through a metal fastener or anchor may also damage tissue at locations away from the surgical site.